As adhesive tape for fixing lead pins of a lead frame of a semiconductor device, adhesive tape, wherein polyacrylonitrile resin, epoxy resin, phenol resin, or acrylonitrile-butadiene resin adhesive layer is applied alone or in combination onto a supporting film such as polyimide, is used conventionally. Such adhesive tape is employed where low heat resistant property of adhesive does not cause trouble. However, in recent years advance within the technology in the semiconductor field, greater heat resistance has been required in the abovementioned adhesive. For example, when ceramic material is used as sealing material for semiconductor chips, glass that has low melting point is used for sealing by heating it to a temperature of 450.degree. to 500.degree. C. In this case, adhesives currently in use have problems of poor heat resistance, surface contamination of components due to gas produced during heating, and occurrence of pin holes from out-gassing of the low melting point glass part. Moreover, on exposure to high temperature, the adhesives lose most of their bonding strength to fix the lead pins and the deformation of the lead pins by heat occurs.
Technology for producing semiconductor devices has advanced at an everincreasing speed. Semiconductor devices are highly refined, and integrated, and propagation speed of signals in the semiconductor devices is accelerated more and more. With such advances, capacity, speed and compactness of each of computers from a super computer to a personal computer are being improved rapidly. Various kinds of electronics also have the same tendency as the computers. The abovementioned highly integrated and accelerated semiconductor devices can process more and more data, and accelerate propagation of signals in the devices. However, the propagation of signals between the devices is slow in ordinary nonintegrated packaging, which slows down the propagation speed of signals in the whole electronic system.
In order to improve performance, new technology such as multichip module, wherein plural semiconductor devices are put together on a substrate, finds practical use for speedy propagation of signals between semiconductor devices, and at the same time, for compact mounting of semiconductor devices.
For example, a multilayer interconnection substrate is constructed from copper, aluminum, or other conductor material and polyimide as insulator material. The multilayer interconnection substrate is required to have layer insulation property. A surface of the top layer where the interconnection of conductors is exposed is over-coated with polyimide precursor (polyamic acid) solution to prevent contamination caused by being exposed to air, and to prevent conductors from oxidating. Forming a protecting film on the top layer in this way not only costs more, but also makes the production process complex and less efficient.
It has been proposed to form the above-mentioned protection film by a process which comprises the steps of applying polyimide precursor solution as by a spin coater and conducting heat treatment at a high temperature of 300.degree. to 350.degree. C. in nitrogen flow, and also by a method using polyimide paste, which is prepared so that a screen printing is possible. Although the screen printing results in less wasted resin than the spin coat method, it is necessary in screen-printing to conduct a heat treatment at the high temperature of 350.degree. C. in nitrogen flow. The films have also been produced by applying conventional low heat resistant adhesive on aromatic polyimide film to form an adhesive layer, and pasting it on the top layer of a multilayer interconnection substrate. However, just as in the case of the above-mentioned ceramic packaging, this method also has a problem with respect to the heat resistance of the adhesive even though the polyimide film has heat resistant property. Thus, as yet there is no adequate adhesive film for producing multilayer interconnection substrate or for sealing by ceramic packaging.
Furthermore, when electronics are produced, semiconductor devices and multilayer interconnection substrates are subjected to heavy heat loads. For example, there are deposition of wiring metal such as aluminum and copper, die bonding, gold streak bonding, sealing by using low melting point glass or epoxy resin, soldering when said semiconductor devices are mounted to the electronics, etc. If the adhesive of the adhesive film employed in those processes could have excellent workability at low temperature and be able to adhere easily by low pressure, and could also have high heat resistance, reliability of the obtained electronics would be improved. Moreover, with improvement of heat resistance of the adhesive, it would be possible to adopt a new production process giving the electronics new characteristics, thereby providing added value and productivity to the electronics.
However, since the above-mentioned adhesives for general purpose such as polyacrylonitrile resin, and epoxy resin have low glass transition point, heat resistant adhesive strength cannot be expected at all. Therefore, to solve the problem in heat resistance, the Tg of adhesives must be raised. For example, in epoxy resin, by introducing a high heat resistant component into the molecular structure of the epoxy resin, selecting a main material with high crosslinking density and curing agent, and conducting heat curing at high temperature, the Tg can be raised and some degree of heat resistant strength can be achieved. However, even in such a method, adhesiveness is shown at a temperature around 200.degree. C. at most, and cannot resist the temperature over 200.degree. C. Also, for example, when adhesives currently in use are exposed to the temperature of 200.degree. C. for a long period, or a high temperature of over 300.degree. C. for a short period, heat deterioration or heat decomposition of the polymer itself is brought about, so that it is impossible to employ the adhesives currently in uses on the abovementioned condition.
Accordingly, if aromatic polyimide with high heat resistance could be used, the adhesive would be able to resist the heat history of over 200.degree. C. for a long period, and the above-mentioned problem will be solved. However, for linear polyimide precursor (linear polyamic acid) conventionally in use wherein an aromatic ring is bonded at a conventional para position and the Tg is high, voids are liable to occur in the adhesive layer since water is eliminated when the linear polyimide precursor is changed to the imide, and complete elimination of solvent is difficult because the linear polyimide precursor dissolves only in high boiling point solvents and voids are liable to occur. Also, since the high Tg linear polyimide precursor is essentially non-thermoplastic, and not fluid when adhered, it is hard to employ it as an adhesive.
Further, a polyimide adhesive [SAMPE Quarterly, 1981, Vol. 13, page 20] has been proposed wherein the Tg of linear polyimide precursor which has an aromatic ring bonding at a meta position is lowered to 260.degree. C. in order to introduce thermoplasticity, and hot melt type adhesion is conducted after being changed to imide in order to prevent occurrence of voids. This polyimide adhesive shows, for example, adhesive strength of over 100kgf/cm.sup.2 even at a high temperature of 200.degree. C. when iron is adhered to another iron, but requires a temperature over 320.degree. C. as condition to conduct adhesion. Since the Tg is about 260.degree. C., molecular chains are fluid and adhesive strength is very low when temperature is over 260.degree. C.
Moreover, by using polyetherimide as adhesive, which is linear heat resisting polymer with low Tg (Tg=217.degree. C.) , the temperature required for bonding can be lowered because Tg is low. However, the lower the Tg is, the lower heat resistant adhesive strength is at high temperature.
Heat resistant adhesive material has been proposed which comprises applying polyimide added with thermoplasticity on one side or both sides of rigid nonthermoplastic polyimide film used for flexible print substrates. The heat resistant adhesive material is produced by applying thermoplastic polyimide precursor (polyamic acid) solution to non-thermoplastic polyimide film, drying up solvent of the solution, and heating it to the temperature over Tg of said thermoplastic polyimide precursor to thereby change it to imide.
Moreover, denatured polyimide, wherein flexible diamine with an aromatic ring bonding at a meta position and acid anhydride are substituted for rigid diamine of monomer with aromatic ring bonding at a para position and acid anhydride to thereby change a molecular structure of the above-mentioned linear polyimide and Tg is changed, loses its heat resisting adhesive strength at the temperature over Tg of this polyimide. Also, when conducting adhesion, a temperature higher than Tg by about 50.degree. to 100.degree. C. is needed and high pressure is required because melt viscosity gets higher when pressing.
It is therefore an object of the present invention to provide adhesive material which can adhere at low temperature and by low pressure, and at the same time, have excellent heat resisting bonding strength.